A standard mammogram consists of a set of two dimensional (2-D) x-ray images obtained in at least two different projections. The images formed using x-ray mammography are the result of differing x-ray absorption by the different tissues and structures of the body. The image formed is only two-dimensional. If multiple x-ray absorbing structures lie in the same x-ray path, information about some of these structures are likely to be obscured, distorted or both. Additionally, because mammography comprises two-dimensional imaging of a three-dimensional object, summation artifacts, that is the summation of the x-ray absorption of two or more, structures, may appear as false positive findings.
Diagnostically significant features on mammograms include, clustered microcalcifications, and masses. Because a mammogram is a two dimensional image of a three dimensional object, visual illusions can be created in the 2-D images. Specifically, objects such as micro-calcifications that are physically distant, for example, further than 1 cm apart in three dimensional space may appear to be close together because of the collapsing of the third dimension in the 2-D image. This result is visually similar to the constellation effect where stars in the sky appear to be part of a planar figure, such as the Big Dipper, but in fact, are widely separated in terms of radial distance from the Earth. Similarly, areas of the breast may appear to be radiographically dense and mass-like because of the summation of x-ray absorption by physically distinct structures in spatially separated planes perpendicular to the x-ray beam.
Physicians interpret mammograms in accordance with recommendations by the American College of Radiology, and standards specified by the Mammography Quality Standards Acts. Currently, mammographic interpretation is both subjective and qualitative. Radiologists apply judgment to identify lesions and potential lesions in a mammographic view and to determine the concordance or discordance of any such findings between mammographic views. Current methods for establishing concordance or discordance of lesions include: (a) evaluating the number of microcalcifications in each view; (b) evaluating the size of a group or groups or a cluster or clusters of microcalcifications in each view; (c) evaluating the radiographic density of lesions in each view; and (d) evaluating the shape and the uniformity of microcalcifications, masses or both in each view. These subjective and qualitative criteria are only somewhat effective in correctly establishing the concordance or discordance of candidate lesions and thus decrease the accuracy of mammography. Current methods including those described above are limited by the displacement of lesions due to the physical compression of the breast during the performance of the mammogram.
While not a replacement for the experienced radiologist, computer aided diagnosis (CAD) systems are designed to increase efficiency and reduce error. CAD software for mammography is available today. Such software typically analyzes each of the 2-D mammography views independently. While existing solutions are sensitive, in other words, they identify almost all features of diagnostic interest, they are not sufficiently specific as they too often identify false positive findings. Many false-positive findings associated with prior art approaches are constellation effect, summation artifacts, or the like, resulting from analyzing each 2-D mammogram view in isolation.
The absence of quantitative methods and automated tools for establishing the spatial concordance of a lesion or lesions between two or more mammographic or mammography views results in a higher than desired occurrence of false positive findings for both physician evaluated and computer evaluated mammography. False positive findings add significant costs to the health care system and almost always result in added stress and anxiety for the patient. False positive results can cause patients to be subjected to unnecessary follow-up testing which is costly, uncomfortable and emotionally stressful. False positive findings may result in unnecessary medical intervention, including tissue sampling, such as cytology, biopsy, or surgical excision. Furthermore, false positive CAD findings represent a potential for future litigation if the interpreting physician is not careful to adequately document his or her findings related to the CAD findings. This extra documentation reduces physician productivity and profitability. Since interpretation of mammograms requires the interpreting physician to use the breast imaging reporting and data system (BIRADS) lexicon, an incorrect BIRADS category may be assigned, resulting in an inappropriate follow-up interval, an unnecessary patient recall, or a missed or delayed diagnosis of breast cancer.